Implement number prefix parsing (/[\+\-]+/)

Fixes #535

parser.cpp

@@ -941,6 +941,22 @@ namespace Sass {
     }
   }
 
+  // parse +/- and return false if negative
+  bool Parser::parse_number_prefix()
+  {
+    bool positive = true;
+    while(true) {
+      if (lex < block_comment >()) continue;
+      if (lex < number_prefix >()) continue;
+      if (lex < exactly < '-' > >()) {
+        positive = !positive;
+        continue;
+      }
+      break;
+    }
+    return positive;
+  }
+
   Expression* Parser::parse_map()
   {
     To_String to_string(&ctx);
@@ -1203,6 +1219,10 @@ namespace Sass {
     else if (lex< sequence< not_op, spaces_and_comments > >()) {
       return new (ctx.mem) Unary_Expression(pstate, Unary_Expression::NOT, parse_factor());
     }
+    else if (peek < sequence < one_plus < alternatives < spaces_and_comments, exactly<'-'>, exactly<'+'> > >, number > >()) {
+      if (parse_number_prefix()) return parse_value(); // prefix is positive
+      return new (ctx.mem) Unary_Expression(pstate, Unary_Expression::MINUS, parse_value());
+    }
     else {
       return parse_value();
     }

parser.hpp

@@ -234,6 +234,7 @@ namespace Sass {
     Simple_Selector* parse_pseudo_selector();
     Attribute_Selector* parse_attribute_selector();
     Block* parse_block();
+    bool parse_number_prefix();
     Declaration* parse_declaration();
     Expression* parse_map_value();
     Expression* parse_map();

prelexer.cpp

@@ -147,6 +147,18 @@ namespace Sass {
       return sequence< exactly<'-'>, exactly<'-'>, identifier >(src);
     }
 
+    // Match number prefix ([\+\-]+)
+    const char* number_prefix(const char* src) {
+      return alternatives <
+        exactly < '+' >,
+        sequence <
+          exactly < '-' >,
+          optional_spaces_and_comments,
+          exactly< '-' >
+        >
+      >(src);
+    }
+
     // Match interpolant schemas
     const char* identifier_schema(const char* src) {
       // follows this pattern: (x*ix*)+ ... well, not quite

prelexer.hpp

@@ -396,6 +396,8 @@ namespace Sass {
     const char* quoted_string(const char* src);
     // Match interpolants.
     const char* interpolant(const char* src);
+    // Match number prefix ([\+\-]+)
+    const char* number_prefix(const char* src);
 
     // Whitespace handling.
     const char* optional_spaces(const char* src);